Gantry machines are often used in industry for moving a payload over a large area. Typically a gantry machine includes a transverse member which is used to support a payload which may move along the transverse member or be fixed in location on the transverse member. The transverse member is supported on a pair of spaced apart longitudinal members defying a longitudinal direction. FIG. 1 shows a gantry machine as known in the prior art. A payload 10 rides on a transverse member 12 and moves along an axis labeled as Axis P3. The transverse member 12 is supported on spaced apart longitudinal members 14 and 16. Motors labeled motor 1 and motor 2 are provided to move the ends of the transverse member 12 to move the payload in the longitudinal direction. Thus, using currently available state-of-the-art technology, the gantry system shown in the FIG. 1 is controlled along three separate axes P1, P2 and P3. Physical axes P1 and P2, controlled by motor 1 and motor 2, are given the same command or used in a "master/slave" arrangement. In such an arrangement, P2 will blindly follow P1 in the "slave" mode. A problem with this prior art configuration is that the two axes P1 and P2 are similar or identical and use similar or identical controllers. Movements along any one of these axes is unknown to the other. Since these controlled axes have essentially the same bandwidth, proper movement along either P1 or P2 appears as a disturbance to the other axis. These two axes will therefore "crosstalk" to each other and cause poor performance.
A prior art solution is to detune the controllers so that P1 and P2 move relatively slowly and therefore tend not to disturb one another. Another option, as disclosed in U.S. Pat. No. 4,812,725 is to close the loop on only one motor, for example the motor on axis P1, and leave the motor controlling axis P2 in an open loop mode. In this case, a control loop operates on P1 and a motor command is generated for P2 so that it is proportional to P1. This configuration will eliminate the crosstalk between the two controllers but results in a loss of accuracy due to having two degrees of freedom and allowing one of these degrees of freedom to be uncontrolled. Essentially, the angle of the transverse member of the gantry is free to be any quantity limited only by the mechanical guidance provided by the transverse member. It is therefore desirable to have two closed-loop controllers for each of the axes P1 and P2 but nonetheless eliminate the disturbance crosstalk problem.